Expose hideBacklogList and hideClosedList on EpicBoard type in GraphQL

Similarly to the regular BoardType for issue boards, we need to expose hideBacklogList and hideClosedList on Epic boards.

These are boolean values on the boards_epic_boards table, so can probably just be exposed in the EE::EpicBoardType.

For example:

diff --git a/ee/app/graphql/types/boards/epic_board_type.rb b/ee/app/graphql/types/boards/epic_board_type.rb
index 5169887e3cd..04e0b2728ad 100644
--- a/ee/app/graphql/types/boards/epic_board_type.rb
+++ b/ee/app/graphql/types/boards/epic_board_type.rb
@@ -21,6 +21,12 @@ class EpicBoardType < BaseObject
             description: 'Epic board lists.',
             extras: [:lookahead],
             resolver: Resolvers::Boards::EpicListsResolver
+
+      field :hide_backlog_list, type: GraphQL::BOOLEAN_TYPE, null: true,
+        description: 'Whether or not backlog list is hidden.'
+
+      field :hide_closed_list, type: GraphQL::BOOLEAN_TYPE, null: true,
+        description: 'Whether or not closed list is hidden.'
     end
   end
 end